Dietary fat is transported in the chicken by portomicrons; these large lipoproteins enter the portal blood of the small intestinal villi. We have shown by electron microscopy that avian portomicrons resemble mammalian chylomicrons in size, but their mode of transport differs. Portomicrons enter the intestinal blood vessels through endothelial intracytoplasmic vesicles, whereas chylomicrons enter the intestinal lymphatics through gaps between endothelial cells. We have also shown that the sinusoidal endothelium of the chicken liver, like that of the mammal, is fenestrated. Because the fenestrae are relatively few in number, the endothelium is less porous in the chicken than in the rat. We postulate that this prevents the hepatocytes from being swamped by dietary fat, but makes the chicken susceptible to diet-induced atherosclerosis.